1. Field of the Invention
The present invention relates to a thermoplastic fluorine-containing copolymer having flexibility.
2. Discussion of Background
A thermoplastic fluorine-containing copolymer having flexibility and excellent moldability is desired in the field of tubes, sealing materials, films or composite materials thereof. A copolymer of tetrafluoroethylene (hereinafter referred to as TFE) and propylene, which contains at most 60 mol % of polymer units based on TFE, is known as an elastomer.
A three-component copolymer comprising TFE, polypropylene and vinylidene fluoride (hereinafter referred to as VDF) is known as an elastomer.
Further, a copolymer of a fluorinated comonomer of the formula (2) or (3) given hereinafter with TFE or VDE is known as an elastomer.
For example, JP-A-50-50488 discloses a three-component copolymer comprising from 50 to 65 mol % of polymer units based on TFE, from 20 to 45 mol % of polymer units based on propylene and from 5 to 26 mol % of polymer units based on VDF.
JP-A-52-44895 discloses a three-component copolymer comprising from 5 to 50 mol % of polymer units based on TFE, from 5 to 40 mol % of polymer units based on propylene and from 30 to 90 mol % of polymer units based on VDF.
Further, JP-A-52-45685 discloses a three-component copolymer comprising from 25 to 50 mol % of polymer units based on TFE, from 25 to 45 mol % of polymer units based on propylene and from 10 to 30 mol % of polymer units based on VDF.
JP-A-58-71906 discloses a copolymer comprising from 50 to 88 mol % of polymer units based on a fluoroolefin such as TFE or VDF and from 12 to 50 mol % of polymer units based on the comonomer of the formula (3) as defined in the present invention.
Further, JP-A-1-22908 discloses a three-component copolymer comprising from 30 to 80 mol % of polymer units based on TFE, from 5 to 60 mol % of polymer units based on the copolymer of the formula (2) as defined in the present invention, and from 3 to 50 mol % of polymer units based on the comonomer of the formula (3) as defined in the present invention. Such a copolymer is described to be a resilient copolymer having excellent low temperature characteristics.
These copolymers are excellent in flexibility. However, their molded products are elastomeric and, as such, can not practically be used unless they are vulcanized.
On the other hand, a two-component copolymer comprising TFE and VDF is known to be a thermoplastic resin having a melting point of from 150.degree. to 300.degree. C., but it does not have flexibility (Polymer Science USSR A18, No. 12, p2691-2699).
Accordingly, it is conceivable that by increasing the content of polymer units based on TFE and decreasing the content of polymer units based on propylene in a three-component copolymer comprising TFE, propylene and VDF, it may be possible to obtain a copolymer having a thermoplastic nature while maintaining flexibility.
In fact, a copolymer comprising from 40 to 80 mol % of polymer units based on TFE, from 5 to 25 mol % of polymer units based on propylene and from 10 to 55 mol % of polymer units based on VDF, has been confirmed to be a thermoplastic fluorine resin which has flexibility and which has a melting point of from about 130.degree. to about 200.degree. C.
A thermoplastic resin is usually required to have a low melt viscosity for extrusion. However, with the above three-component copolymer, it has been found that if it has a low melt viscosity, the mechanical properties, particularly the tensile strength and the tensile elongation at a high temperature, deteriorate.